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Module 9 Final Quiz: I/O Systems & Disk Scheduling MCQs
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1
What is Direct Memory Access (DMA)?
A.
The CPU directly accessing L1 cache
B.
A specialized hardware controller that transfers entire blocks of data between I/O devices and main memory without continuous CPU intervention
C.
A software API for accessing databases
D.
A method to bypass file permissions
Explanation & Key
Correct Answer:
2
When is Polling (Busy-Waiting) preferable to Interrupt-driven I/O?
A.
When the I/O device is extremely slow (e.g. keyboard input)
B.
When the I/O device is extremely fast and data arrives in microsecond intervals, making interrupt-handling and context-switching overhead higher than brief polling
C.
When the system is running on battery power
D.
Polling is never used in modern computing
Explanation & Key
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3
What component constitutes the largest portion of physical disk access latency on a traditional spinning Hard Disk Drive (HDD)?
A.
Transfer Time
B.
Seek Time (the physical mechanical movement of the read/write head arm to the target track)
C.
Rotational Latency
D.
Bus transfer time
Explanation & Key
Correct Answer:
4
What is the primary danger of the Shortest Seek Time First (SSTF) disk scheduling algorithm?
A.
Excessive head movement
B.
Starvation of requests located far from the current head position if a steady stream of requests arrives near the head
C.
Slow sequential throughput
D.
Inability to handle read requests
Explanation & Key
Correct Answer:
5
How does the SCAN (Elevator) disk scheduling algorithm operate?
A.
It scans memory for viruses
B.
The disk arm moves in one direction across tracks servicing requests along the way until it hits the end of the disk, then reverses direction servicing requests on the way back
C.
It services requests strictly in the order they arrived
D.
It jumps to random tracks to balance temperature
Explanation & Key
Correct Answer:
6
How does Circular SCAN (C-SCAN) improve upon basic SCAN?
A.
It moves the head in circles around the platter
B.
It services requests in one direction only; when it reaches the end, it immediately returns to the beginning without servicing requests on the return trip, providing more uniform wait times
C.
It eliminates seek time completely
D.
It doubles disk capacity
Explanation & Key
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7
Consider a disk with 200 cylinders (0-199). Head is at 53. Requests: 98, 183, 37, 122, 14, 124, 65, 67. Under FCFS, what is the total head movement?
A.
236 cylinders
B.
640 cylinders
C.
310 cylinders
D.
500 cylinders
Explanation & Key
Correct Answer:
8
What is RAID 0 (Striping) primarily designed for, and what is its major risk?
A.
High redundancy; risk is slow write performance
B.
High read/write performance by striping data across multiple disks in parallel; risk is zero fault tolerance (failure of any single disk destroys all data)
C.
Mirrored backup; risk is 50% capacity loss
D.
Parity checking; risk is slow controller speed
Explanation & Key
Correct Answer:
9
How does RAID 5 achieve fault tolerance with high storage efficiency?
A.
By storing a full duplicate copy of every drive (100% overhead)
B.
By distributing block-level parity information across ALL member drives, tolerating a single disk failure with an overhead of only $1/N$ of total capacity
C.
By dedicating one central disk solely to parity (RAID 4)
D.
By using hardware encryption
Explanation & Key
Correct Answer:
10
Why do SSDs (Solid State Drives) not require traditional disk scheduling algorithms like SCAN or SSTF?
A.
SSDs do not have files
B.
SSDs have no moving mechanical actuator arms or rotational platters; random access across flash memory cells occurs in uniform time ($O(1)$ electrical latency)
C.
SSDs are managed by the BIOS, not the OS
D.
SSDs only support sequential reads
Explanation & Key
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